1. Field of the Invention
This invention relates generally to apparatus for charging a battery and more particularly, to a charging control system suitable for uniformly charging a plurality of serially-connected battery cells.
2. Description of the Related Art
Recently, industrial robots and electric vehicles using battery cells as power sources are actively developed. Conventionally, lead battery is exclusively used in this field of application, but in recent years, attempts are made to use Ni-Cd battery, Li battery and Ni-H battery, etc. with high energy density. This type of battery generally has an output power of 2.0 to 4.2 V per unit cell. In the case of mounting battery cells on an electric vehicle, a required number of battery cells are serially connected with one another, to thereby derive a desired voltage.
When a battery array comprised of serially-connected battery cells is charged, it is a common practice to pass a preset current between the two electrodes of the battery array so as to charge each of the battery cells. However, in addition to the presence of variations in the characteristics of individual battery cells caused in the manufacturing process, there occurs differences in their internal resistances due to different heat radiating conditions caused by differences in the positions where the battery cells are disposed in the battery array. Thus, all the serially-connected battery cells are not uniform in their characteristics (battery capacities).
If the battery cells are charged by passing a current between the two terminals of the battery array in spite of the above fact, a battery cell having a small recharging capacity, i.e., low performance, is excessively charged, whereas a battery cell having a large recharging capacity, i.e., high performance, is insufficiently charged. Thus, if the charging and discharging operations of the battery array are repeatedly effected, even the characteristic of a battery cell of high performance is degraded and the service lives of all of the serially-connected battery cells are shortened.
To solve these problems, a technique for equalizing the charging voltages of the battery cells is disclosed in, e.g., Japanese Patent Application KOKOKU Publication No. 6-28491 in which charge regulators each connected to a corresponding one of serially-connected battery cells are provided, and a charging current is diverted from a battery cell concerned to an associated one charge regulator according to the charging voltage of the concerned battery cell.
In order to prevent a particular one or ones of the battery cells from being fully discharged, the technique disclosed in the above Publication is operable to supply each battery cell with a trickle charging current of, e.g., 300 mA corresponding to a self-discharging current of the battery cell. To regulate the trickle charging current, resistance values of impedance circuits respectively connected in parallel with the battery cells are changed, to thereby continuously change electric currents flowing through the impedance circuits according to the charging voltages of the battery cells.
In the case of a battery array mounted on an electric vehicle or the like, the battery array is ordinarily charged when the remaining capacity of the battery array is considerably lowered, and hence a charging current of several tens of ampere is to be supplied to the battery array. From this point of view, it is not practical to carry out the diversion or bypass control for such a large charging current based on the technique disclosed in the above Publication since the load loss in the impedance circuits becomes extremely large in that case.
Conventionally, therefore, a measure is taken in which serially-connected battery cells constituting a battery array are simultaneously charged independently of one another by use of a battery charger array which is connected in series with the battery array, as disclosed in Japanese Patent Application KOKAI Publication No. 8-33219, for example. The battery charger array disclosed therein includes battery chargers which are the same in number as the battery cells. Each battery charger includes a rectifier for converting an AC voltage supplied from an AC power source into a DC voltage, a charging voltage supplying unit connected with the rectifier, and a control section for controlling the charging voltage according to the temperature of the battery cell. This system has such a drawback that it requires a relatively large number of component parts.